1. Technical Field
This invention relates to the design of reciprocating pistons, and more particularly to wet lubricated piston and cylinder assemblies that have reduced friction.
2. Discussion of the Prior Art
It is conventional for reciprocating piston engines to splash lubricating oil along the sides (skirt) of the piston and use piston rings to separate such oil from the combustion zone. Depending on the reciprocating speed of the piston, the piston skirt design can have considerable length and barrel circumference such as the long depending skirts used with diesel engine pistons operating at slower speeds. It is thought by the prior art that such skirts more effectively reduce the slapping forces imparted, by an articulating piston rod and induce a surrounding film of lubricant oil operating in the hydrodynamic friction regime (fluid sliding past other fluid layers in a gradient pattern). Accordingly, it is the conventional wisdom of those skilled in the art that redesign of piston skirts in an oil lubricated assembly will not significantly reduce friction losses, since the hydrodynamic lubrication regime provides the lowest possible friction, and that the existing true barrel design promotes such regime.
Our recent studies show this not to be the case, and that conventional piston skirt design is a major contributor to friction losses, and that the oil film operates in the boundary layer or mixed lubrication regime and not substantially in the hydrodynamic friction regime resulting in high friction data results.
Not fully understanding the nature of piston friction, the prior art had earlier tried fluorinated carbon as a reducer of friction in an oil-fed piston system (see U.S. Pat. Nos. 2,817,562 and 4,831,977). Such use of fluorinated carbon has not been commercially successful because it does not promote hydrodynamic film formation and, as a consequence, friction will remain high at high engine speeds causing the coating to have limited life due to wear.
Solid film lubricants have also been tried in reciprocating assemblies where there is essentially no oil lubrication supply (see U.S. Pat. Nos. 3,896,950 and 4,872,432). These trials used graphite and MoS.sub.2 in resin coatings to reduce solid contact friction. However, the modifications that must be made to such solid film lubricants in an oil-lubricated piston system is not known.